Solid oxide fuel cell

ABSTRACT

The solid oxide fuel cell of the present invention has a substrate ( 1 ); an electrolyte ( 3 ) that is disposed on one surface of the substrate ( 1 ); and at least one electrode element E having an anode ( 5 ) and a cathode ( 7 ) disposed on the same surface of the electrolyte ( 3 ) with a predetermined space therebetween.

CROSS REFERENCE TO RELATED APPLICATION

This application is a Divisional Application of prior application Ser.No. 10/561,789, with 371(c) date of Mar. 15, 2007, now U.S. Pat. No.8,101,316, which was a §371 National Stage Application ofPCT/JP04/009347, filed on Jun. 25, 2004, which is hereby incorporated byreference.

TECHNICAL FIELD

The present invention relates to a fuel cell, specifically, to a solidoxide fuel cell that stably generates electricity mainly in a mixed gasof fuel gas and oxidizing gas.

BACKGROUND OF THE INVENTION

Planar-type, tubular-type and other types of cell designs have beenconventionally proposed for solid oxide fuel cells.

A planar-type cell comprises an anode and a cathode disposed on thefront and back surfaces, respectively, of a flat electrolyte. Athus-formed cell is used in a condition where a plurality of such cellsare laminated having an interconnector (separator) between adjacentcells. The interconnectors (separator) connect cells in series or inparallel, and completely separate the fuel gas supplied to each cellfrom the oxidizing gas. A gas seal is provided between each cell andseparator (for example, Japanese Unexamined Patent Publication No.1993-3045). However, in this planar-type cell, because the gas seal isprovided by applying pressure to the cell, the cell is easily damaged byoscillation, heat cycles, etc. This poses a significant problem inbringing the fuel cell to practical use.

In contrast, a tubular-type cell disclosed in, for example, JapaneseUnexamined Patent Publication No. 1993-94830, comprises an anode and acathode disposed on the external surface and internal surfacerespectively, of a tubular electrolyte. Among tubular-type cells,vertical stripe-type and horizontal stripe-type fuel cells have beenproposed. Although a tubular-type fuel cell is advantageous in havingexcellent gas-sealing properties, its production is complicated becauseits construction is more complex than that of a planar type cell andthis makes the construction cost thereof high.

Furthermore, these cell designs have the following drawbacks: both inplanar-type cells and tubular-type cells, the electrolyte needs to bethin to improve performance, and the ohmic resistance of the electrolytematerial needs to be reduced. However, an unduly thin electrolyte lackssufficient strength and decreases the vibration resistance anddurability of the cell.

For this reason, a non-diaphragm-type solid oxide fuel cell has beenproposed to take the place of the above-mentioned planar-type andtubular-type fuel cells, wherein, as disclosed in, for example, JapaneseUnexamined Patent Publication No. 1996-264195, an anode and a cathodeare arranged on the same surface of a solid electrolyte substrate, andelectricity is generated by supplying a mixed gas of fuel and oxidizinggas. Because fuel gas and oxidizing gas do not need to be separated inthis fuel cell, a separator and gas seal become unnecessary, and theconstruction and the production thereof can be significantly simplified.

In a non-diaphragm-type solid oxide fuel cell, because an anode and acathode are formed in the vicinity of each other on the same surface ofa solid electrolyte and conduction of oxygen ions occurs mainly on thesurface of the electrolyte, the thickness of the electrolyte does notsignificantly effect the cell performance as it does in planar-type ortubular-type cells. Therefore, the electrolyte may be thickened whilemaintaining the same level of cell performance, and this can reduce itsvulnerability to damage.

As described above, in prior-art solid oxide fuel cells, thevulnerability to damage is alleviated by thickening the electrolyte.However, because in many cases only those portions in the vicinity ofthe surface of the electrolyte contribute to the cell reaction, cellperformance will not be significantly improved even if the electrolyteis thickened. Therefore, thickening the electrolyte merely increases itsproduction costs.

The present invention aims to solve the above problem and provides asolid oxide fuel cell that can alleviate the vulnerability to damage,reduce its production costs, and obtain high power output.

DISCLOSURE OF THE INVENTION

The first solid oxide fuel cell of the present invention has beendeveloped to solve the above problem. The solid oxide fuel cellcomprises a substrate; an electrolyte that is disposed on one surface ofthe substrate; and at least one electrode element comprising an anodeand a cathode that are disposed on the same surface of the electrolytewith a predetermined space therebetween.

It is preferable that the fuel cell further comprises anotherelectrolyte disposed on the other surface of the substrate, and anelectrode element which comprises an anode and a cathode that aredisposed on the same surface of this electrolyte formed on the othersurface of the substrate, with a predetermined distance therebetween.

A plurality of electrode elements may be disposed on each surface of thesubstrate using electrolyte. These electrode elements may be connectedto one another using an interconnector disposed on the fuel cell. It isalso possible to provide an interconnector on the side of a device towhich the fuel cell is to be disposed so that these electrode elementscan be connected to one another by the interconnector when the fuel cellis installed.

It is preferable that a groove be formed in the electrolyte so as toseparate adjacent electrode elements from each other. The groove may beformed so as to cut through the electrolyte and reach the substrate.

It is also possible to partition the electrolyte between adjacentelectrode elements. In this case, it is preferable that an insulatingmaterial be disposed between adjacent electrolytes. This arrangementeases the connection between electrode elements using an interconnector,and reliably separates the electrolytes from each other.

In the fuel cell, it is preferable that the electrolyte be formed byprinting. Alternatively, the electrolyte may be formed into a plate-likeor sheet-like shape, and adhered to the substrate via adhesive.

In the fuel cell, it is preferable that electrode elements be formed insuch a manner that one of the electrodes is surrounded by the otherelectrode with a predetermined space therebetween.

The second solid oxide fuel cell of the present invention comprises aplurality of single cells each having an electrolyte, an anode, and acathode, the solid oxide fuel cell further comprising a substrate forsupporting the plurality of single cells, and wherein the electrolyte ofeach single cell is disposed on the substrate so as to have apredetermined space therebetween.

A plurality of cells may be arranged on each surface of the substrate.These cells may be connected to one another using an interconnectordisposed on the fuel cell. It is also possible to provide aninterconnector on the side of a device to which the fuel cell is to bedisposed so that these cells can be connected one another by theinterconnector when the fuel cell is installed.

In this fuel cell, it is preferable that the electrolyte be formed byprinting. It is also possible to form the electrolyte into a plate-likeshape and attach the electrolyte to the substrate via adhesive.

In each of the above-explained fuel cell, it is preferable that thesubstrate be formed from a ceramic material.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a partially expanded sectional view of a fuel cell accordingto the first embodiment of the present invention.

FIG. 2 is a schematic plan view of the fuel cell of FIG. 1.

FIG. 3 illustrates one example of the procedure for producing the fuelcell of FIG. 1.

FIG. 4 shows a fuel cell according to the second embodiment of thepresent invention, wherein (a) is a partial cross-sectional view and (b)is a schematic plan view.

FIG. 5 illustrates one example of the procedure for producing the fuelcell of FIG. 4.

FIG. 6 shows a fuel cell according to the third embodiment of thepresent invention, wherein (a) is a partial cross-sectional view and (b)is a schematic plan view.

FIG. 7 illustrates one example of the procedure for producing the fuelcell of FIG. 6.

FIG. 8 illustrates an example of the procedure for producing the fuelcell of the third embodiment.

FIG. 9 is a cross-sectional view of another example of the fuel cell ofthe present invention.

FIG. 10 is a plan view of the still another example of the fuel cell ofthe present invention.

FIG. 11 is a cross-sectional view of another example of the fuel cell ofFIG. 6.

FIG. 12 is a plan view of the still another example of the fuel cell ofthe present invention.

FIG. 13 is a partially expanded sectional view of the fuel cell of FIG.12.

FIG. 14 shows another example of the fuel cell of FIG. 6, wherein (a) isa cross-sectional view and (b) is a schematic plan view.

FIG. 15 shows a fuel cell of Example 1 of the present invention, wherein(a) is a plan view and (b) is a cross-sectional view.

FIG. 16 shows a fuel cell of Example 3 of the present invention, wherein(a) is a plan view and (b) is a cross-sectional view.

FIG. 17 is a cross-sectional view of a fuel cell of Example 4 of thepresent invention.

BEST MODE FOR CARRYING OUT THE INVENTION First Embodiment

Hereunder, a first embodiment of the solid oxide fuel cell of thepresent invention is explained with reference to the drawings. FIG. 1 isa partial cross-sectional view of a fuel cell of the present embodimentand FIG. 2 is a schematic plan view of the fuel cell.

As shown in FIGS. 1 and 2, this fuel cell comprises a sheet-likesubstrate 1, and an electrolyte 3 laminated on one surface of thesubstrate 1, wherein a plurality of electrode elements (single cells) Eeach comprising a pair of anode 5 and cathode 7 is disposed on the samesurface of the electrolyte 3. In each electrode element E, an anode 5and a cathode 7 are formed into a strip-like shape and arranged to havea predetermined space therebetween. The distance between the anode 5 andthe cathode 7 is preferably 1 μm to 500 μm, and more preferably 10 μm to500 μm.

As described above, a plurality of electrode elements E are formed onthe electrolyte 3 and these electrode elements E are connected to oneanother in series via an interconnector 9. In other words, the cathode 7of each electrode element E is connected to the anode 5 of an adjacentelectrode element E via an interconnector 9.

The materials for the fuel cell having the above structures areexplained below. It is preferable that the substrate 1 be formed of amaterial having excellent adhesiveness to the electrolyte 3. Specificexamples of usable materials are SUS, as well as alumina-basedmaterials, silica-based materials, titania-based materials and the likeceramic-based materials. Ceramic-based materials having excellent heatresistance of at least 1000° C. are particularly preferable. Note thatthe thickness of the substrate 1 is preferably not less than 50 μm.

Known materials for solid oxide fuel cell electrolytes may be used asthe material for the electrolyte 3. Specific examples of usablematerials include oxygen ion-conductive ceramic materials such asceria-based oxides doped with samarium, gadolinium, and/or the like,strontium- and/or magnesium-doped lanthanum gallate-based oxides,scandium and/or yttrium-containing zirconia-based oxides, etc. Thethickness of the electrolyte 3 is preferably 10 μm to 5000 μm, and morepreferably 50 μm to 2000 μm.

The anode 5 and the cathode 7 may be formed from a ceramic powdermaterial. The average particle diameter of such a ceramic powder isgenerally 10 nm to 100 μm, preferably 50 nm to 50 μm, and morepreferably 100 nm to 10 μm. The average particle diameter can bemeasured, for example, in accordance with JISZ8901.

The anode 5 may be formed from a mixture of a metal catalyst and ceramicpowder comprising an oxide ion conductor. Examples of usable metalcatalysts are those that are stable in reducing atmospheres and exhibithydrogen oxidizing activity, such as nickel, iron, cobalt, noble metals(platinum, ruthenium, palladium, etc.), etc. Oxide ion conductors havinga fluorite or perovskite structure are preferably used. Examples ofoxide ion conductors having a fluorite structure are ceria-based oxidesdoped with samarium, gadolinium, and/or the like, scandium and/oryttrium-containing zirconia-based oxides, etc. Examples of oxide ionconductors having a perovskite structure are strontium- and/ormagnesium-doped lanthanum gallate oxides. Among the above materials, itis preferable to form the anode 4 from a mixture of an oxide ionconductor and nickel. To prepare the mixture, a ceramic materialcontaining an oxide ion conductor and nickel may be physically mixed, ornickel may be modified with a ceramic powder. The above-mentionedceramic materials may be used singly or as a combination of two or moresuch materials. The anode 5 may be formed from a single metal catalyst.

Metal oxides of Co, Fe, Ni, Cr, Mn, etc., having a perovskite structuremay be used as a ceramic powder material for the cathode 7. Specificexamples thereof include oxides such as (Sm, Sr) CoO₃, (La, Sr) MnO₃,(La, Sr) CoO₃, (La, Sr)(Fe, Co)O₃, (La, Sr)(Fe, Co, Ni)O₃, etc. Amongthose, (La, Sr) MnO₃ is particularly preferable. Such ceramic materialsmay be used singly or as a combination of two or more such materials.

The anode 5 and the cathode 7 are formed by using the above materials asmain ingredients and adding appropriate amounts of binder resin, organicsolvent, etc. To be more specific, it is preferable that binder resinand the like be added in such a manner that the content of the mainingredients is 50 to 95 wt. %. The thickness of the cathode 3 and theanode 5 after sintering is preferably 1 μm to 500 μm, and morepreferably 10 μm to 100 μm.

As with the anode 5 and the cathode 7, the electrolyte 3 is formed byusing the above materials as main ingredients and adding suitableamounts of binder resin, organic solvent, etc. In the mixture of themain ingredients and binder, it is preferable that the content of themain ingredients be not less than 80 wt %. It is also possible tosubject the powder comprising the above-mentioned materials to uniaxialpress molding and cold isostatic pressing (CIP), sinter the resultant ata predetermined temperature for a predetermined period of time, and thencut the resultant into a plate-like or sheet-like shape having desirablethickness and other dimensions. The thus-obtained plate-like orsheet-like shaped electrolyte 3 is attached to the substrate 1 viaadhesive, thus obtaining a fuel cell. Note that when the electrolyte 3is formed by printing, it is preferable that a stress relaxation layerformed from an adhesive material whose coefficient of thermal expansionis between that of the substrate 1 and the electrolyte 3 be disposedbetween the substrate 1 and the electrolyte 3. This prevents cracking ofthe thin film during sintering due to the differences in thecoefficients of expansion of the substrate 1 and the electrolyte 3.

In a fuel cell having the above-described structure, power is generatedin the following manner: a gas mixture of fuel gas containinghydrocarbons, such as methane and ethane, together with air or a likeoxidizing gas is supplied to one surface of a single cell C at a hightemperature (for example, 400° C. to 1000° C.). This initiates ionicoxygen conduction in the electrolyte 3 that is sandwiched between theanode 5 and the cathode 7, thus generating electric power. In a fuelcell having the above-described structure, those portions other than thevicinity of the surface of the electrolyte 3 do not significantlycontribute to the cell reaction, and therefore the production costs canbe decreased by making the electrolyte 3 thin to an extent that does notadversely affect the cell performance. In the fuel cell of the presentembodiment, because the electrolyte 3 is supported on the substrate 1,even when the electrolyte 3 is a thin film, high resistance tooscillation and heat cycles can be maintained.

By connecting a plurality of electrode elements E in series using aninterconnector 9, high voltage output can be achieved. Theinterconnector 9 can be formed of conductive metals such as Pt, Au, Ni,Ag, Cu, SUS, metal materials, or lanthanum chromite-based materials suchas La (Cr, Mg)O₃, (La, Ca)CrO₃, and (La, Sr)CrO₃. Such materials can beused singly or as a combination of two or more such materials. It isalso possible to add additives such as binder resin described above.

Furthermore, the interconnector 9 may be formed on the electrolyte 3 viaan insulating layer. In this case, it is preferable that the materialfor the insulating layer be a ceramic-based material as these haveexcellent heat resistance. Specific examples of usable ceramic-basedmaterials are alumina-based materials, silica-based materials,titania-based materials and like ceramic-based materials. By arrangingthe interconnector 9 on the electrolyte 3 via an insulating layer,electrical contact between the interconnector 9 and the electrolyte 3can be prevented. This arrangement has the following advantage. If theinterconnector is formed on the electrolyte to connect adjacentelectrode elements as in conventional techniques, the interconnectorexhibits electrical conductivity and, sometimes, ion conductivitysimilar to that observed in electrode reactions, and may function in thesame manner as an electrode. This may reduce the intrinsic open circuitvoltage of the fuel cell. In contrast, in the structure of the presentembodiment, because the interconnector 9 and the electrolyte 3 are notin electrical contact with each other, reduction of open circuit voltagecan be prevented. This also prevents the open circuit voltage frombecoming unstable, and achieves desirable output characteristics.

One example of a method for producing the above-described fuel cell isexplained below with reference to FIG. 3. First, electrolyte paste,anode paste, and cathode paste are prepared by using the above-describedpowder materials for the electrolyte 3, anode 5, and cathode 7 as mainingredients, and mixing these pastes with appropriate amounts of binderresin, organic solvent, etc. The viscosity of each past is preferablyabout 10³ mPa·s to 10⁶ mPa·s, which is desirable for conducting screenprinting described latter. In the same manner, binder resin and/or otheradditives are added to the above-described powder material to prepareinterconnector paste. The viscosity of the interconnector paste is thesame as that mentioned above.

Second, the electrolyte paste is applied on the substrate 1 by screenprinting, and dried and sintered at a predetermined temperature for apredetermined time period, thus formed the electrolyte 3 (FIG. 3( a)).Subsequently, the anode paste is applied to a plurality of portions onthe electrolyte so as to have strip-like shapes by screen printing, andthen the paste is dried and sintered at a predetermined temperature fora predetermined time period, forming a plurality of anodes 5 (FIG. 3(b)). Subsequently, the cathode paste is applied to portions facing theanodes 5 by screen printing, and the paste is dried and sintered at apredetermined temperature for a predetermined time period, forming aplurality of electrode elements C (FIG. 3( c)). In the last step, theinterconnector paste is linearly applied between the electrode elementsC by screen printing so that the plurality of electrode elements C areconnected to one another in series by the interconnector 9. Theinterconnector 9 is thus formed (FIG. 3( d)).

In the above-described fuel cell, because electrolyte lies between theadjacent electrode elements, this electrolyte may function as a paththrough which oxygen ions migrate during generation of electric power.Therefore, the electrolyte between the electrode elements together withthe anode and the cathode sandwiching the electrolyte may form a fuelcell and generate electric power. In this structure, the open circuitvoltage that is inherent in a single cell and the open circuit voltagegenerated between single cells cancel each other and therefore a shortcircuit occurs inside the cell. It is believed that this reduces theopen circuit voltage of a fuel cell as a whole. Therefore, even if thenumber of the electrode elements is increased, the open circuit voltageas a whole may not be equal to the “open circuit voltage per electrodeelement times the number of electrode elements”. The second embodimentof the present invention that was developed taking this drawback intoconsideration is explained below.

Second Embodiment

A solid oxide fuel cell of the second embodiment of the presentinvention is explained below. FIG. 4 shows the fuel cell of the presentembodiment wherein (a) is a side elevational view and (b) is a planview. Here, a fuel cell comprising two electrode elements is explained.

As shown in FIG. 4, this fuel cell comprises a sheet-like substrate 1and an electrolyte 3 formed on one surface of the substrate 1, whereintwo electrode elements E each having an anode 5 and a cathode 7 pair aredisposed on the same surface of the electrolyte 3. The structure of eachelectrode element E is the same as in the first embodiment. A groove Vis formed between the electrode elements E to partition them. A cathode7 in one electrode element E₁ is connected to an anode 5 in the adjacentelectrode element E₂ by an interconnector 9 so as to cross the groove V.A portion of the interconnector 9 is inserted in the groove V.

The materials for the substrate 1, electrolyte 3, anode 5, cathode 7,and interonnnector 9 used in the present embodiment are the same asthose used in the first embodiment, and therefore detailed explanationis omitted here. The method for generating electrical power of thepresent embodiment is also the same as that of the first embodiment.

As described above, in the present embodiment, a groove V, whose depth Dis greater than the thickness R of the electrolyte 3 beneath the groove,is formed in the electrolyte 3 between the electrode elements E₁,E₂ (forexample, D=800 μm, R=200 μm). This reduces the path in the electrolyte 3between the electrode elements E₁,E₂ through which oxygen ions migrate.As a result, generation of electrical power is minimized, and thereforereduction of the voltage is prevented. Note that the width of groove Vis preferably 1 μm to 5000 μm, as described in the third embodiment.

A method for producing the fuel cell is explained with reference to FIG.5. The electrolyte paste, anode paste, cathode paste, and interconnectorpaste used in the present embodiment are the same as in the firstembodiment. As shown in FIG. 5( a) to FIG. 5( c), an electrolyte 3,anodes 5, and cathodes 7 are formed on the substrate 1. The productionprocedure until here is the same as that in the first embodiment.

A groove V is then formed in the electrolyte substrate 3 between theelectrode elements E₁,E₂ (FIG. 5( d)). The groove V may be formed by,for example, blasting, laser beam machining, cutting, etc. Aninterconnector 1 is then formed by applying interconnector paste betweenthe anode 5 in the electrode element E₂ and the cathode 7 in theelectrode element E₁ as shown in FIG. 5( e), obtaining the fuel cellshown in FIG. 4.

In this embodiment, the path through which oxygen ions migrate isreduced by providing a groove in the electrolyte between the electrodeelements, and therefore electrical power generation between theelectrode elements is reduced. However, it is also possible tocompletely partition the electrolyte between the electrode elementsconnected by the interconnector. Such an embodiment is explained below.

Third Embodiment

A solid oxide fuel cell of the third embodiment of the present inventionis explained below with reference to the drawings. FIG. 6 shows the fuelcell of the present embodiment, wherein (a) is a partial cross-sectionalview and (b) is a schematic plan view.

As shown in FIG. 6, this fuel cell comprises a sheet-like substrate 1and a plurality of single cells C (in FIG. 6, two single cells C₁,C₂)disposed on one surface of the substrate 1. The single cells C areconnected in series via an interconnector 9.

Each single cell C comprises a rectangular electrolyte 3 disposed on onesurface of the substrate 1 and an anode 5 and a cathode 7 pair disposedon the same surface of the electrolyte 3. The electrolyte 3 of eachsingle cell C is located so as to be at a predetermined distance fromthe electrolyte 3 of the adjacent single cell C so that a gap S isformed between the electrolytes 3. The gap is preferably, for example,10 μm to 5000 μm, and more preferably 10 μm to 500 μm. The anode 5 andthe cathode 7 on the electrolyte 3 are formed into strip-like shapes,and arranged so as to have a predetermined space therebetween. Thedistance L between the anode 5 and the cathode 7 is preferably 1 μm to5000 μm, and more preferably 10 μm to 500 μm. As shown in FIG. 2, acurrent collector member 8 is provided on each of the end electrodes ofthe fuel cell, i.e., the anode 5 of one single cell C₁ and the cathode 7of the other single cell C₂.

As described above, the interconnector 9 connects adjacent single cellsC. Specifically, the interconnector 9 connects a cathode 7 of one singlecell C₁ to an anode 5 of the other single cell C₂. In this structure,the interconnector 9 is formed on the electrolyte 5, and disposed on thesubstrate 1 between the adjacent single cells C so as to cross over thegap S.

The materials for the substrate 1, electrolyte 3, anode 5, cathode 7,and interconnector 9 used in the present embodiment are the same as inthe first embodiment, and therefore detailed expiation is omitted here.The method for generation electrical power is also the same as that ofthe first embodiment. Note that the material for the current collectormember 8 is the same as that for the interconnector.

As described above, in the fuel cell of the present embodiment, becausethe electrolyte 3 is supported by the substrate 1, even when theelectrolyte 3 is a thin film, high resistance against oscillation andheat cycles can be maintained. In the above-explained fuel cell, eachsingle cell C is arranged separately having gaps therebetween andconnected via an interconnector 9. In this embodiment, because noelectrolyte 3 exists between the single cells C, migration of oxygenions between the single cells C is prevented, and formation of a fuelcell between single cells can be prevented. As a result, reduction ofthe open circuit voltage of the fuel cell is prevented, and thereforehigh output can be obtained.

One example of a method for producing the above-described fuel cell isexplained below with reference to FIG. 7. First, electrolyte paste,anode paste, and cathode paste are prepared by using the above-mentionedpowder materials for the electrolyte 3, anode 5, and cathode 7 as mainingredients, and adding and mixing each paste with suitable amounts ofbinder resin, organic solvent, etc. The viscosity of each paste ispreferably about 10³ mPa·s to 10⁶ mPa·s, which is desirable for screenprinting described latter. In the same manner, interconnector paste isprepared by adding binder resin and/or other additives to powdermaterials. The viscosity of the interconnector paste is the same as thatof the paste mentioned above.

Second, the electrolyte paste is applied to a plurality of portions ofthe substrate 1 by screen printing, and dried at predeterminedtemperature for a predetermined time period. A plurality of rectangularelectrolytes 3 having predetermined gaps S between each other are thusformed (FIG. 7( a)). Subsequently, anode paste is applied to eachelectrolyte 3 by screen printing so as to have, strip-like shapes, anddried and sintered at a predetermined temperature for a predeterminedtime period, forming anodes 5 (FIG. 7( b)). Cathode paste is thenapplied by screen printing to each electrolyte 3 in regions facing theanodes 5, and dried and sintered at a predetermined temperature for apredetermined time period, thus forming cathodes 7. A plurality ofsingle cells C are thus formed (FIG. 7( c)). In the last step, aninterconnector 9 is formed by linearly applying interconnector pastebetween single cells C by screen printing so that the plurality ofsingle cells C are connected to one another in series. In thisembodiment, the interconnector 9 is formed so as to cross the gap Sbetween the electrolytes 3 and to pass immediately above the substrate1. Current collector members 8 are provided at the ends of theinterconnector 9. By the above procedure, production of the fuel cell iscompleted (FIG. 7( d)). When a plurality of single cells are formedusing a photosensitive polymer as binder resin, a plurality of singlecells or electrolytes having a desirable pattern can be obtained by thefollowing method. After applying and drying paste, the paste is exposedto light using a mask so as to have a plurality of patterns, theunexposed portions are removed, and the remaining portions are thensintered.

Embodiments of present invention are explained above; however, thepresent invention is not limited to these embodiments and variousmodifications can be made as long as such modifications do not adverselyaffect the present invention. For example, in the production methods ofthe above-described embodiments, screen printing is employed forapplying each paste; however, it is also possible to employ doctor bladecoating, spray coating, lithography, electrophoretic deposition, rollcoating, dispenser coating, CVD, EVD, sputtering, and transfer printing,as well as other typically used printing methods. Isostatic pressing,oil hydraulic pressing, and other typically used pressing methods may beemployed as post-printing processes.

When an electrolyte is formed by employing an above-mentioned printingmethod, it is preferable to provide a stress relaxation layer betweenthe substrate 1 and the electrolyte 3. Such a stress relaxation layer isformed from an adhesive material having a coefficient of thermalexpansion between that of the substrate 1 and the electrolyte 3. Thisprevents cracking in the electrolyte during sintering due to differencesin coefficients of expansion between the substrate 1 and the electrolyte3.

Alternatively, it is also possible to obtain a fuel cell by preparing aplate-like or sheet-like shaped electrolyte, and attaching it to asubstrate using adhesive. In this case, in particular when a fuel cellof the third embodiment is formed, a fuel cell can be obtained byattaching each of a plurality of electrolytes of single cell havingpredetermined dimensions. Alternatively, it is also possible to obtain afuel cell by attaching an electrolyte to a substrate, and partitioningthe electrolyte into single cells by cutting. For example, as shown inFIG. 8, a plurality of single cells C can be formed by attaching theelectrolyte 3 to the substrate 1, providing electrodes 5 and 7 (FIG. 8(a)), and forming a groove V that cuts through the electrolyte 3 andreaches the substrate 1 so as to partition the electrolyte 3 (FIG. 8(b)).

In the above embodiments, the electrolyte 3, the anode 5, and thecathode 7 are formed only on one surface of the substrate 1; however, itis also possible to provide an electrolyte 3, an anode 5, and a cathode7 on the other surface of the substrate 1 as shown in FIG. 9. Note thatFIG. 9( a) to FIG. 9( c) correspond to the first to third embodiments,respectively. An example of a method for producing such fuel cells issuch that, during forming the electrolyte 3, the anode 5 and the cathode7 on one surface of the substrate 1, another electrolyte, anode andcathode are also formed on the other surface of the substrate 1 in thesame manner, and two cells having the same structure disposed one oneach surface of the substrate 1 are thus formed. This arrangement makesit possible to obtain high output (electric power) while maintaining thecompactness of the fuel cell.

In the above embodiments, a plurality of electrode elements E or singlecells C are connected in series via an interconnector 9; however, it isalso possible to connect them in parallel. For example, as shown in FIG.10( a), in the first embodiment, the interconnector 9 may connect ananode in one electrode element E to an anode in the other electrodeelement E and a cathode 7 in one electrode element E to a cathode 7 inthe other electrode element E. Alternatively, it is also possible toincorporate both series and parallel circuits. By such combination,desirable voltage and electric current can be obtained. Needless to say,it is also possible to form a fuel cell using a single electrode elementE rather than a plurality of electrode elements E.

It is also possible to form gaps between adjacent electrolytes 3, and,as shown in FIG. 11, an insulating film 10 may be disposed in the gap Sbetween electrolytes 3. This allows adjacent electrolytes 3 to bepartitioned by the insulating film 10, electrically separating singlecells C from each other in a more reliable manner, and making theconnection via the interconnector 9 easier. Therefore, formation of afuel cell between single cells C can be reliably prevented, obtaininghigh output.

In this structure, it is preferable that the insulating film 10 beformed from a ceramic-based material. Examples of usable ceramic-basedmaterials are alumina-based and silica-based ceramic materials. As withthe electrolyte, etc., the particle diameter of the ceramic materialpowder forming the insulating film 10 is generally 10 nm to 100 μm andpreferably 100 nm to 10 μm. The insulating film 10 is formed by using aceramic material powder as main ingredients and adding suitable amountsof binder resin, organic solvent, etc. As with the electrolyte, etc.,the thickness of the insulating film 10 after sintering is generally 1μm to 500 μm, and preferably 10 μm to 100 μm.

In the above embodiments, the electrodes are formed into strip-likeshapes, and the anode and the cathode are aligned alternately; however,the shape of the electrode is not limited to this, and the followingarrangement may also be employed. As shown in FIGS. 12 and 13, such afuel cell comprises 24 electrode elements E and these electrode elementsE are connected to one another via interconnectors 9.

Each electrode element E comprises an anode 5 and a cathode 7, wherein aframe-like anode 5 is disposed around a rectangular cathode 7 with apredetermined space therebetween. The external shape of the anode 5 isrectangular in correspondence with the rectangular cathode 7. In thisarrangement, the distance between the anode 5 and the cathode 7 ispreferably 1 μm to 1000 μm, and more preferably 10 μm to 500 μm. Currentcollector members 51 and 71 for outputting electric current are formedon the anode 5 and the cathode 7, respectively. Each current collectormember 51 on an anode 5 is connected to the current collector member 71on a cathode 7 in the adjacent electrode element E by an interconnector9, thereby connecting the electrode elements E in series. Note that thedistance between adjacent electrode elements E is preferably 10 μm to5000 μm and more preferably 1000 μm to 3000 μm.

Each interconnector 9 has the configuration as shown in FIG. 13. Asshown in FIG. 13, between the current collector members 51 and 71 at theends of the interconnector (i.e. crossover section), an insulating layer11 is formed over the anode 5, the cathode 7, and the electrolyte 1. Theinterconnector 9 is formed on the insulating layer 11. Theinterconnector 9 thereby passes over the anode 5 but does not shortcircuit the anode.

The above structure makes integration of circuits easier, and thereforehigh electrical power output can be obtained. The shapes of the fuel andcathodes are not limited to rectangular and they may be formed into, forexample, circular or polygonal shapes.

In the third embodiment, the electrolyte 3 is formed on the substrate 1;however, it is also possible to employ the following arrangement. Asshown in FIG. 14, two concave portions 11, which are rectangular as seenin plan view, are formed in one surface of the substrate 1, and theelectrolyte 3 of a single cell C is placed in each concave portion 11.In this arrangement, each electrolyte 3 is separated from each other bya wall 14 between the concave portions 13. The depth of each concaveportion is preferably 5 μm to 5 mm. If the depth is less than 5 μm, itis difficult to dispose the electrolyte 3 in such a manner that theelectrolyte 3 does not overflow the concave portion 13. If the depththereof is greater than 5 mm, the portion that does not contribute tocell reaction in the electrolyte 3 increases, which increases productioncosts.

In this fuel cell, because the electrolyte 3 of each single cell C isdisposed in a concave portion 13 in the substrate 1, electrolytes 3 areseparated from each other by walls 11 formed between the concaveportions 13. Because the electrolytes 3 are not connected to each otherbetween the adjacent single cells C, it is possible to prevent reductionof open circuit voltage caused by the electrolyte between adjacentelectrodes functioning, as observed in conventional techniques, as apath through which oxygen ions migrate. As a result, high output can beobtained.

Note that the figures show that the interconnectors in some of the aboveembodiments are attached to side surfaces of the electrodes; however, itis also possible to structure the interconnectors so that each end ofthe interconnector is placed on top of each electrode.

The present invention is explained in more detail below.

Example 1

A solid oxide fuel cell as shown in FIG. 15 was manufactured. FIG. 15(a) is a plan view of the fuel cell of Example 1 and FIG. 15( b) is across-sectional view.

GDC (Ce_(0.9)Gd_(0.1)O_(1.9)) powder (particle diameter of 0.05 μm to 5μm, average particle diameter of 0.5 μm) was used as an electrolytematerial and mixed with cellulose-based binder resin to obtain anelectrolyte paste (weight ratio of the electrolytematerial:cellulose-based binder resin was 95:5). By diluting the pasteusing a solvent, the viscosity of the electrolyte paste was set to 5×10⁵mPa·s, as is desirable for screen printing.

Furthermore, anode paste was prepared as the anode material by mixingNiO powder (particle diameter of 0.01 to 10 μm, average particlediameter of 1 μm) and SDC (Ce_(0.8)Sm_(0.2)O_(1.9)) powder (particlediameter of 0.01 μm to 10 μm, average particle diameter of 0.1 μm) insuch amounts that the weight ratio of NiO powder:SDC powder in themixture was 7:3, and cellulose-based binder resin was added to themixture in such an amount that the resultant anode paste comprised 80wt. % of the mixture. In other words, the ratio of mixture:binder resinwas 80:20. By diluting using a solvent, the viscosity of the anode pastewas set to 5×10⁵ mPa·s, as is desirable for screen printing.

Cathode paste was prepared as the material for the cathode by mixing SSC(Sm_(0.5), Sr_(0.5), CoO₃) powder (particle diameter of 0.1 μm to 10 μm,average particle diameter of 1 μm) with cellulose-based binder resin insuch amounts that the cathode paste comprised 80 wt. % of the SSCpowder. In other words, the weight ratio of SSC power:binder resin inthe resultant cathode paste was 80:20. As with the anode, by dilutingusing a solvent, the viscosity of the cathode paste was set to 5×10⁵mPa·s, as is desirable for screen printing. The substrate 1 was made ofan alumina-based substrate 10 mm square with a thickness of 1 mm.

The electrolyte paste was applied onto the substrate 1 by screenprinting to 10 mm square area, dried at 130° C. for 15 minutes, andsintered at 1500° C. for 10 hours, obtaining an electrolyte 3 having athickness after sintering of 200 μm.

The anode paste was applied so as to have a width of 500 μm and a lengthof 7 mm by screen printing. The paste was dried at 130° C. for 15minutes and sintered at 1450° C. for one hour, obtaining an anode 5having a thickness after sintering of 30 μm. Subsequently, the cathodepaste was applied by screen printing on the same surface of theelectrolyte 3 to which the anode paste had been applied. The cathodepaste was applied so as to have a width of 500 μm, length of 7 mm, anddistance from the anode of 500 μm. As with the anode, the cathode pastewas dried at 130° C. for 15 minutes and sintered at 1200° C. for onehour, thus forming a cathode 7 having a thickness after sintering of 30μm, accordingly, obtaining a solid oxide fuel cell comprising a singleelectrode element.

The thus-produced solid oxide fuel cell of Example 1 was subjected tothe following evaluation test. Specifically, a mixture gas of methaneand oxygen was introduced to the fuel cell at 800° C., causing thereaction CH₄+ 1/20₂→2H₂+CO. The anode 5 comprising a nickel oxide wasthus reduced, and the current/voltage characteristics thereof were thenevaluated. It is also possible to introduce hydrogen gas instead of theabove-described mixture gas to conduct reduction treatment.

As a result, it was confirmed that a solid oxide fuel cell that canobtain a maximum power density of 65 mW/cm² was produced in Example 1.

Example 2

Example 2 is explained below. Example 2 differs from Example 1 in that astress relaxation layer lies between the electrolyte and the substrate.In Example 2, the stress relaxation layer paste was prepared by mixingGDC and Al₂O₃ powder (particle diameter of 0.1 to 10 μm, averageparticle diameter of 3 μm) in such a manner that the weight ratio ofGDC:Al₂O₃ powder became 50:50. The stress relaxation layer paste wasdiluted by solvent so as to have a viscosity that is suitable for screenprinting, i.e., about 5×10⁵ mPa·s.

Detailed explanations of other materials are omitted as these were thesame as in Example 1.

The production process is explained below. The stress relaxation layerpaste was applied on the substrate so that the paste had an appliedthickness of 30 μm, and dried at 130° C. for 15 minutes. Thereafter, theelectrolyte, the anode, and the cathode were formed in that order inExample 1.

The thus-formed fuel cell had reduced cracking in the thin filmelectrolyte compared to a fuel cell without a stress relaxation layer.With regard to the cell performance, as in Example 1, the fuel cell ofExample 2 obtained a maximum power density of 65 mW/cm².

Example 3

In Example 3, the solid oxide fuel cell shown in FIG. 16 wasmanufactured. The same materials for the substrate, electrolyte, andelectrodes as in Example 1 were used. Au powder (particle diameter of0.1 μm to 5 μm, average particle diameter of 2.5 μm) was used as thematerial for the current collector member and interconnector connectingsingle cells. Cellulose-based binder resin was added to the Au powder,preparing the interconnector paste and current collector member paste.The viscosity of the interconnector paste was set to 5×10⁵ mPa·s, as isdesirable for screen printing.

Subsequently, the electrolyte paste was applied on the substrate 1 byscreen printing so that a plurality of rectangular electrolytes wereformed. The electrolyte paste was patterned so that two rectangularelectrolytes positioned 0.5 mm from the edge of the substrate and eachhaving dimensions of 9×4.2 mm were placed with a distance of 0.6 mmtherebetween. The electrolyte paste was dried at 130° C. for 15 minutesand sintered at 1500° C. for 10 hours, forming the electrolyte 3 havinga thickness after sintering of 200 μm. Thereafter, the anode paste wasapplied to each electrolyte 3 by screen printing in such a manner thatan anode 5 having a width of 500 μm, length of 7 mm, and appliedthickness of 50 μm was formed on the electrolyte 3. The anode paste wasdried at 130° C. for 15 minutes and sintered at 1450° C. for one hour,obtaining an anode having a thickness after sintering of 30 μm.Subsequently, the cathode paste was applied on the same surface of theelectrolyte 3 to which the anode paste had been applied by screenprinting in such a manner that a cathode 7 having a width of 500 μm,length of 7 mm, applied thickness of 50 μm, and a distance from theanode 5 of 500 μm was formed on each electrolyte 3. As with the anode 5,the cathode paste was then dried at 130° C. for 15 minutes and sinteredat 1200° C. for one hour. Its thickness after sintering was 30 μm.

The interconnector paste was then applied by screen printing (width of 2μm, thickness of 50 μm), the single cells C were connected in series asshown in FIG. 16, and current collector members 8 were formed on theelectrodes of the cells at each end of the serial connection. The solidoxide fuel cell of Example 3 was thus obtained.

A fuel cell of Comparative Example 1, which is compared to that ofExample 3, was manufactured in the following manner. In ComparativeExample 1, a 10×10 mm electrolyte with a thickness of 1 mm was preparedand used as a substrate. Two each of anodes and cathodes having the samedimensions as those in Example 3 were formed on the electrolyte with thesame distances therebetween as in Example 3 and connected in seriesusing an interconnector. A fuel cell comprising a single cell was alsoprepared as Comparative Example 2.

The thus-obtained fuel cells of Example 3 and Comparative Example 1 weresubjected to an evaluation test as described below. A mixed gas ofmethane and oxygen was introduced to the fuel cell at 800° C. to causethe reaction CH₄+ 1/20₂→2H₂+CO so that the anode 5 comprising nickeloxide was reduced. The current/voltage characteristics were thenevaluated. Note that, to conduct reduction treatment, hydrogen gas maybe introduced instead of the above-described mixed gas.

The results show that the open circuit voltage of the fuel cell ofComparative Example 2, which comprises a single cell, was 610 mV, andthe open circuit voltage of the fuel cell of Example 3, which comprisestwo cells, was 1190 mV. The fuel cell of Comparative Example 1, whichcomprises two pairs of electrodes, had 900 mV of open circuit voltage.From these results, it was confirmed that the open circuit voltage inthe fuel cell of Comparative Example 1 was not twice that of the fuelcell of Comparative Example 2, due to short circuiting occurring insidethe cell. In contrast, in Example 3, because the electrolytes wereplaced with a predetermined distance therebetween, short circuit in thecell was reduced. Therefore, the fuel cell of Example 3 produced almosttwice the open circuit voltage of the fuel cell of Comparative Example2.

Example 4

In Example 4, an insulating film was placed between each single cell inthe fuel cell shown in FIG. 16. This arrangement allows adjacentelectrolytes 3 to be separated from each other by the insulating film 10as shown in FIG. 17, and therefore the single cells can be electricallyseparated from each other in a more reliable manner. Furthermore, thisarrangement makes the connection of the interconnector 9 easier and morereliable. Accordingly, formation of a fuel cell between single cells canbe reliably prevented, and therefore high electrical power output can beachieved.

In this case, it is preferable that the insulating film 10 be formedfrom a ceramic-based material. Examples of usable ceramic-basedmaterials are alumina-based and silica-based ceramic materials. As withthe electrolyte, the particle diameter of the ceramic material powderforming the insulating film 10 is generally 10 nm to 100 μm, andpreferably 100 nm to 10 μm. The insulating film 10 is prepared using anabove-mentioned ceramic material powder as a main ingredient whileadding suitable amounts of binder resin, organic solvents, etc. As withthe electrolyte, etc., the thickness of the insulating film aftersintering is generally 1 μm to 500 μm and preferably 10 μm to 100 μm.

The same electrolyte paste, anode paste, cathode paste, and substrate asin Example 3 were prepared. Au powder (particle diameter of 0.1 μm to 5μm, average particle diameter of 2.5 μm) was used as the material forthe current collector member and the interconnector connecting eachsingle cell. Interconnector paste and current collector member pastewere prepared by adding cellulose-based binder resin to the Au powder.The viscosity of the interconnector paste was 5×10⁵ mPa·s, which isdesirable for screen printing. Insulating film paste for forming theinsulating film was also prepared by adding cellulose-based binder resinto alumina powder (alumina particle diameter of 0.1 to 10 μm).

Subsequently, the insulating film paste was applied to the substrate 1in the portion which will be between the electrolytes 3, sintered at1800° C., thus forming the insulating film 10. Electrolyte 3, anodes 5,and cathodes 7 were formed in the same manner as in Example 3. Here, theelectrolytes 3 were positioned so as to sandwich the insulating filmpaste therebetween. As in the Example 3, each single cell C wasconnected in series using the interconnector 9, and a current collectormember 8 was then provided at each end electrode of the serialconnection, thus forming the solid oxide fuel cell of Example 4.

The fuel cell of Example 4 was also evaluated by the same method asExample 4, and exhibited the same characteristics as in Example.

INDUSTRIAL APPLICABILITY

The present invention provides a solid oxide fuel cell, which alleviatesvulnerability to damage, reduces production costs, and obtains highelectrical power output.

1. A solid oxide fuel cell comprising: a substrate; an electrolytedisposed on one surface of the substrate; and at least one electrodeelement comprising an anode and a cathode disposed on the same surfaceof the electrolyte and with a predetermined space therebetween, whereinthe electrolyte is formed by printing.